1. Field of the Invention
The present invention relates to an image display apparatus for dealing with moving-images, and in particular, for example, to an active matrix drive type of liquid crystal display device.
2. Description of the Related Art
Hitherto liquid crystal displays (hereinafter simply referred to as “LCDs”) have been used widely for various displays in clocks, pocket calculators, word processors, personal computers, navigation systems, etc. by utilizing their features, i.e., low profile, light weight, low power consumption, etc. Before the wide use of the LCDs, cathode ray tubes (hereinafter simply referred to as “CRTs”) were used widely. In comparison with the CRTs, the LCDs have the advantages of having a greatly reduced thickness (depth) and requiring lower power consumption. Furthermore, in comparison with electro-luminescence (EL) devices, plasma display panel (PDP) devices and the like, the LCDs have the advantages of being able to be driven electrically and to attain full color display easily. By making full use of such advantages, the LCDs have been demanded in the fields related to moving-images, for example, personal computers, various monitors, portable television sets, digital video cameras, etc. In accordance with this trend, the LCDs are requested more and more to have higher moving-image performance. However, as yet the display performance of the LCD is far behind that of the CRT in the moving-image performance.
A first factor for making the moving-image performance of the LCD lower than that of the CRT is that the electro-optic response characteristic of its liquid crystal is low, that is, the time response characteristic of the transmittance of the liquid crystal is low. The time response characteristic of the transmittance is hereinafter referred to as “response speed.” The LCDs put into actual use at present are categorized into a twisted nematic type, simply referred to as TN, and a super-twisted nematic type, simply referred to as STN. The response speed of the STN type is about hundred milliseconds. Even the TN type has a low response speed of several tens of milliseconds. Therefore, the response of the liquid crystal is not completed within one frame time of 16.7 ms which is an image information rewriting time in the case of 60 frames per second. In other words, since the response speed of the liquid crystal is low, several frames time is required until the response of the liquid crystal is completed. Therefore, even if image signal rewriting is carried out, images for plural frames are mixed in an image, thereby causing a blurred image. However, in recent years, a liquid crystal capable of responding within one frame time, that is, 16.7 ms, has been developed. It is shown from a moving-image performance test of such a liquid crystal having such a high response speed that an image obtained using the liquid crystal are improved with respect to blurs in comparison with an image obtained by using the TN liquid crystal. However, it is found that the image quality is lower than that of the CRT in sharpness.
Because of the above-mentioned reasons, it is found that, in addition to the response speed of the liquid crystal, there are decisive reasons why the moving-image performance of the LCD is lower than that of the CRT. As for the causes of the low performance, the reports by IBM and NHK point out that the drive system of the LCD differs from that of the CRT. This will be described below.
As described above, even if a liquid crystal having a response speed sufficiently lower than one frame time, the LCD cannot obtain the moving-image performance equivalent to that of the CRT. To clarify this phenomenon, the CRT and the LCD are considered below in view of a light-emitting mode. The CRT is a display device wherein the phosphor at the portion hit by the scanning electron beam emits light momentarily. Therefore, the CRT is a display apparatus of the impulse light-emitting type wherein light is emitted from the phosphor during only part of one frame time. On the other hand, the LCD, the active-matrix type LCD comprising thin-film transistors (TFTs) in particular, is a hold-type display device that keeps holding an image at each pixel until the next rewriting and is continuously illuminated by a back-light or the like.
FIG. 12A shows a change in the light emission intensity of a typical CRT of the impulse light-emitting type with respect to time, and FIG. 12B shows a change in the transmittance of a typical LCD of the active matrix transmission type as a hold-type light-emitting display apparatus. It is believed that this difference in the light-emitting mode makes the moving-image performance of the LCD lower than that of the CRT. In reality, if the LCD is driven in the impulse light-emitting mode, its moving-image performance is improved significantly according to the results of experiments. Therefore, in order to obtain moving-image performance equivalent to that of the CRT, it is found that the LCD must be driven by the impulse-type drive used for the light-emitting mode of the CRT as shown in FIG. 12A, instead of the hold-type drive in the conventional continuous light emission as shown in FIG. 12B.
As a method of carrying out the impulse-type drive to improve the moving-image performance of the LCD, methods proposed in Japanese Unexamined Patent Publication JP-A 64-82019 (1989) and Japanese Unexamined Patent Publication JP-A 11-109921 (1999) are available for example. JP-A 64-82019 proposes a method of intermittently lighting a back-light in synchronization with frame cycles. In this prior art, the back-light of a transmission-type liquid crystal display panel is divided into plural back-light portions capable of being turned on and off selectively, and the divided back-light portions are turned on and off sequentially in synchronization with the drive timing of the scanning electrodes of the liquid crystal display panel to display moving-images. Each back-light portion is turned on immediately after all the image scanning electrodes in the illumination range corresponding thereto are selected, and is turned off in the other periods. As described above, even in the LCD, the impulse-type drive can be carried out by performing image display in only a desired period and by forcibly attaining a non-image state in the other periods. As a result, it is possible to prevent an image from being seen mixed with images for other continuous frames in one screen at a time. Therefore, it is possible to improve the quality and moving-image performance of displayed images.
However, in the above-mentioned method, the non-image state can be controlled only for each back-light portion. Therefore, optimum timing cannot be set for each scanning line. In other words, even if lighting is carrying out at optimum timing for a certain scanning line, the timing is not necessarily optimal for other scanning lines. Furthermore, when the back-light is turned on and off, the optical characteristics of the back-light, that is, the light emission and persistence characteristics thereof, cause problems. The phosphor components included in the back-light are three primary colors, i.e., R, G and B. If the optical characteristics of these three kinds of phosphors are identical with one another in rising and decaying for example, no problem occurs. However, in reality, the optical characteristics of R, G and B are different from one another. As a result, when the back-light is turned on and off, if the persistence characteristic of a color, for example, green, is longer than those of the other colors, coloring (green coloring in this case) occurs. In other words, the quality of display is lowered, although moving-image performance can be improved.
JP-A 11-109921 discloses a method of improving moving-image performance by attaining impulse-type drive while the back-light is always lit, without carrying out the above-mentioned back-light on/off drive. Hereafter, the state wherein the back-light is always lit is referred to as a “continuous light-emitting mode.” In the above-mentioned JP-A 64-82019, the non-image display state is attained by turning off the back-light. On the other hand, JP-A11-109921 discloses a method of attaining the non-image display state by writing a black display signal for example for a constant period after an image is displayed on the liquid crystal display panel. In other words, instead of intermittently turning on the back-light, an image is displayed, and then a non-image used to erase the image once is displayed, thereby attaining impulse-type drive. With this method, impulse-type drive can be carried out while the back-light is always lit. Therefore, impulse-type drive can be carried out while eliminating the disadvantages of the back-light on/off drive.
However, the method of attaining the impulse-type drive by using the non-image state also has a problem, because this method is characterized in that within one frame time an image is displayed and a non-image is also displayed. In other words, two screens must be displayed within one frame time. Therefore, the liquid crystal is required to respond within ½ frame time, and the panel including its drive circuit is required to have double-speed writing design wherein a signal is written within ½ frame time.
FIG. 13A and FIG. 13B show response characteristics in the case where double-speed writing is performed within ½ frame time. FIG. 13A shows a case wherein the liquid crystal responds within ½ frame time. One frame time is halved into an image signal writing time and a non-image signal writing time to attain impulse-type drive. However, in reality, it is very difficult to raise the response speed of the liquid crystal. If this drive is carried out while the response speed of the liquid crystal is insufficient, the charging for image display becomes insufficient as shown in FIG. 13B, and the charging for non-image display also becomes insufficient. Therefore, the moving-image performance is not improved, and the problem of low constant also occurs.
FIG. 14A and FIG. 14B show changes in the case where the image signal writing time is changed and in the case where the non-image signal writing time is changed, respectively. FIG. 14A shows a state wherein the image signal writing time is increased so that an image can be written sufficiently. Since the non-image signal writing time is shortened, only insufficient non-image signal writing can be carried out. As a result, as shown in the figure, complete blackening is not attained, and moving-image performance is lowered. On the other hand, if the non-image signal writing time is extended as shown in FIG. 14B, the image signal writing time is shortened, and image display having sufficient constant cannot be carried out.
As described above, in the case where LCD is driven in a hold-type drive method to display moving-images, the following problems must be solved.
1) Impulse-type drive must be carried out to improve the LCD so as to have a moving-image performance as equal as that of the CRT.
2) The impulse-type drive by turning on and off the back-light causes no contrast reduction, however,
a) lighting timing is different among positions of the display area, whereby the moving-image performance cannot be improved uniformly in the display area, and
b) respective optical characteristics such as light emission and persistence characteristics of the back-light phosphors are different among the three primary colors, i.e., R, G and B, at the current state, whereby the display on the display panel is colored.
3) In the hold-type drive the back-light is continuously in on-state (continuous light-emitting mode). Accordingly, if double-speed writing is carried out in which image signal writing and non-image signal writing thereafter is carried out within one frame cycle, moving-image performance can be improved uniformly in the display area, thereby no undesired coloring of the panel occurs in displaying an image, however, it is difficult at the present state of art that the liquid crystal responds to the signal within ½ frame time. In the case where the liquid crystal does not respond within ½ frame time and therefore the ratio of the image signal writing time is increased, the non-image signal writing time is shortened and as a result the non-image signal is not written completely, whereby moving-image performance is not improved. Needless to say, the ratio of the non-image signal writing time is increased and the image signal writing time is shortened, the image signal is not written completely, whereby only insufficient image display is performed, whereby the contrast of the displayed image is lowered.